The present invention relates generally to the detection of cavitation events in a fluid medium and more specifically to a sensor for monitoring acoustic cavitation in ultrasonic cleaning vessels.
Ultrasonic cleaning vessels are used throughout industry for a range of cleaning applications including sterilisation. Objects to be cleaned are placed within a bath of water containing an acoustic field generated by a single or group of transducers. The transducer will typically produce an acoustic driving field within the frequency range of about 20 kHz to 100 kHz, depending on the items to be cleaned, the degree of cleansing required and so forth.
The mechanical vibrations generated by the acoustic field induce bubble motion within the medium. This arises from small bubbles dissolved within the liquid medium, which are commonly associated with weak points within the liquid such as impurities. Cavitation is the term used to describe the oscillation, expansion and collapse of the bubbles. The oscillation and collapse of the bubbles themselves generates acoustic waves those components cover a range of frequencies dependent on details of the bubble motion but extend well into the MHz frequency range, much higher than the acoustic driving field. The bubbles can also collapse catastrophically generating shock-waves and these, along with water-jetting produced by collapse, are typical mechanisms by which ultrasonic cleaning of the component takes place. The degree of cavitation can be varied by adjusting the magnitude of the electrical drive to the transducers, thereby affecting the acoustic pressures generated in the bath.
The density of cavitating bubbles, the frequency of bubble events and the violence of the collapse is related to the effectiveness of any cleaning technique which utilises this phenomenon. The ability to measure these parameters or a combined effect of the bubble activity would enable optimisation of the driving amplitude of the transducers for a given type of cleaning vessel. It would enable the long-term performance of the cleaning vessel to be monitored and would also provide the basis of a test by which the performance of cleaning vessels could be compared.
The present invention seeks to provide a sensor for the detection of bubble events within a fluid subjected to an acoustic driving field.
According to an aspect of the present invention there is provided a cavitation sensor including an ultrasonically absorbent coating substantially enclosing a piezoelectric element and a conduit; the conduit including a boundary delimited by the piezoelectric element, the ultrasonically absorbent coating being substantially transparent to acoustic driving field frequencies.